1. Field of the Invention
The present invention relates to a polishing pad, a method and an apparatus for treating a polishing pad, and a polishing method; particularly, to a polishing pad and a polishing method used for polishing a semiconductor wafer (hereinafter, it may be simply referred to as "wafer").
2. Description of Related Art
In a process for fabricating a semiconductor integrated circuit, with the progress of larger scale integration of recent semiconductor devices, the requirement for the flatness of a wafer which is a material for the devices becomes stricter. Therefore, it is required to improve the processing accuracy of a polishing process which influences the flatness of the wafer.
In the polishing process, a wafer which was etched by mixed acid which is mixture of hydrofluoric acid, acetic acid and nitric acid or by an aqueous solution of sodium hydroxide, potassium hydroxide or the like, that is, an etched wafer, is a work to be polished. Because the general flatness of the etched wafer is not enough to meet the requirement and the surface roughness thereof is large, three staged polishing are often carried out in the polishing process. The first polishing step is for mirror polishing the etched wafer by removing the waviness thereof, so that the amount of removal, that is, the entire polishing stock removal is large. The third polishing step is for improving the minute surface roughness of the wafer, so that the entire polishing stock removal is small. The second polishing step which is an intermediate step between the first and the third steps has an intermediate purpose therebetween. Therefore, in each polishing step, a polishing is carried out by changing the polishing condition such as the type of the polishing pad, the type and supplying amount of the polishing slurry, the relative rotation speed between the wafer and the polishing pad, and the contacting pressure of the wafer which is contacted with the polishing pad, that is, the polishing pressure, according to the difference of the purpose of the each polishing step.
Among various types of polishing conditions, the one with respect to the polishing pad has the greatest influence on the flatness of the wafer, especially to the waviness which have a cycle or wavelength of several millimeters and to the ripples which have a cycle or wavelength of scores of microns. That is, the polishing pad has the greatest influence in the first polishing step which gives the largest entire polishing stock removal.
The wafer is polished by a polishing machine, for example, the one which is shown in FIG. 4. That is, a wafer 1 is held to adhere on the lower surface of a holding plate 8 which is attached to a top ring 2 of a polishing head. A rotational motion is given to the wafer 1 by a head driving motor 5, while the wafer 1 is pressed against a polishing pad 9 which is adhered on the upper surface of a turn table 4 by an air cylinder 3. On the other hand, to the turn table 4, the rotational motion is also given by a turn table driving motor 6. Accordingly, a relative motion occurs between the wafer 1 and the polishing pad 9. The wafer 1 is polished by supplying polishing slurry 7 to a surface of the polishing pad 9 while giving the relative motion between the wafer 1 and the polishing pad 9. For the polishing slurry in the case, abrasive grains of colloidal silica dispersed in alkali aqueous solution is used, so that the wafer 1 is polished by the so-called mechano-chemical function which compounds a mechanical function and a chemical function.
In the mechano-chemical function, the speed of removing the wafer material, for example, silicon is proportional to the force that the wafer 1 vertically presses the polishing pad 9, that is, the polishing pressure. As a result, when the polishing pressure is non-uniformly distributed within the wafer 1, the removal amount of the material is not uniform within the wafer 1, so that the wafer 1 is not flatly polished. Therefore, in order to obtain the flat wafer 1, it is important to make the distribution of the polishing pressure uniform within the wafer 1.
However, the polishing pad 9 has a visco-elastic property to cause creep deformation. That is, when the constant load is continuously applied to the polishing pad 9, the polishing pad 9 is pressed to be thinned. The amount of the reduced thickness, that is, the amount of displacement is increased rapidly right after applying the load and is increased slowly thereafter. Then, the thickness of the polishing pad 9 does not return to the thickness before the load was applied even if the load is removed, so that the displacement will remain forever. Accordingly, under the constant load, the amount of the displacement of the polishing pad 9 greatly depends on the time during which the polishing pad 9 is subjected to the load.
FIG. 5 shows the relation between the distance from the center 60 of the turn table 4 and the applied time of the polishing load which is applied to the polishing pad 9 in an automatic single wafer polishing machine which polishes the wafer 1. According to FIG. 5, it is understood that the applied time of the polishing load is not uniform according to the position on the polishing pad 9. The problem of non-uniformity of the applied time of the polishing load also occurs when the position of the wafer from the center of the turn table varies in the automatic single wafer polishing machine and in the batch processing polishing machine wherein many wafers are held to adhere on a holding plate.
When the applied time of the polishing load is not uniform according to the position on the polishing pad 9, the amount of displacement of the polishing pad 9 depends on the load application time as above-described, so that the amount of displacement of the polishing pad 9 which is adhered to the turn table 4 is not uniform. The thickness of a polishing pad which was used actually has been measured. As a result, a difference in the amount of displacement of about 50 .mu.m is observed between a portion to which the load was most applied and a portion to which no load was applied.
For the purpose of removing harmful influence on the flatness of the wafer, especially waviness and ripple, and avoiding the influence of the visco-elastic property of the polishing pad, especially, the creep deformation which rapidly progresses right after the polishing pad is used, a treatment of the polishing pad, of which a method is similar to the polishing method of the wafer is carried out.
For the treatment, a polishing machine which is used for polishing the wafer, that is, for manufacturing the wafer is used. The time when the treatment is carried out is right after that a new polishing pad is adhered to the turn table. For tools which are used instead of the wafer are a ceramic disc on which grooves are formed, and a quartz or a silicon disc which is adhered on the surface of the holding plate which is used for actual polishing of the wafer.
However, if the polishing machine which is used for manufacturing the wafer is also used for the treatment of the polishing pad, there is a problem that the manufacturing of the wafer is required to be stopped during the treatment of the polishing pad. For the treatment of the polishing pad, it is required to suitably select the temperature of the polishing pad, the load applied to the polishing pad or the like. However, there is a problem that it is not possible to suitably select the operating conditions except the polishing time in the polishing machine. Further, there is a problem that it is always required to prepare the tools for the treatment of the polishing pad.